1. Field of the Invention
The present invention relates to a device and a method for measuring the volume of a transferable liquid, the density of the liquid, and/or the viscosity of the liquid and converting any one or all of those characteristics into a corresponding frequency signal. More particularly, the present invention relates to a system for applying a signal to a device in contact with the liquid to be characterized, obtaining the resonant response of the device, and transforming that response into a frequency signal that can be utilized to characterize the liquid with great accuracy.
2. Description of the Prior Art
There is a large patent and engineering literature concerning capacitance strain gauges for use in fluid measurements, typically adapted to pressure measurement at minimized sensor volume displacement. Some (e.g. Hewlett Packard Model 1290 Series Quartz Transducer) have been adapted to medical fluid control, sensing pressure across a membrane and obtaining a small deflection of a strain gauge surface that mates with the membrane. In many applications, e.g., with medical liquids and in analytical chemistry and manufacture, accurate knowledge of volume is of primary importance, while only an approximate knowledge of pressure is needed for status monitoring (e.g., to determine whether pressure in the patient IV line is indicative of normal unobstructed flow). In common engineering practice, knowledge of volume is most often obtained by positive-displacement pumping or dispensing, rather than by volume displacement sensing. An inherent disadvantage to measurement by mechanical positive-displacement fluid pumping or dispensing is that the inherent mechanical limitations of the fluid delivery system--stick-slip-stick instability in sliding fluid seals, hysteresis in elastomer fluid seals, backlash in gears and cams--become inherited limitations of the implicit fluid measurement. For precision quantitative measurements, fluid mass (weight) is often used rather than volume, but mass measurement on a scale or balance is vibration-sensitive and requires taking a fluid into an isolated environment, free of tugging tubes, etc. Reading volume from a fluid meniscus has its own limitations in both accuracy and adaptability to automation.
It would be useful to have a strain gauge transducer whose volume compliance is sufficiently large and linear to permit its use for direct measurement of volume, with a perhaps less-accurately controlled pressure calibration based on the pressure/volume characteristic of the device. While many strain gauge pressure transducers are in fact small-volume transducers calibrated in relation to their pressure/volume slope as pressure transducers, these designs have not been adapted to sufficiently large displacements and small pressure/volume slopes to make them useful for most direct volume measurements.
In designing for larger volume displacements, a problem arises with deflection in flat plates. When a plate is nearly flat, its elastic response is approximately linear and determined by the stiffness of the plate in bending, but as larger volumes cause significant curvature in the plate, the combination of curvature and tension in the surface gives rise to a steeply-rising pressure/volume curve, approximating a cube-law term adding significantly to the linear response term even at comparatively small deflections. While a certain amount of consistent, well-characterized nonlinearity is usually acceptable in a measurement system that includes a microprocessor capable of applying a non-linear calibration curve to incoming data, the extreme stiffening of a plate as it becomes curved usually generates too much back pressure at high displacements. Driven to excessive deflection, initially flat plates rupture.
A problem associated with capacitance strain gauges at large deflection is that the capacitance magnitude becomes too small for accurate measurement when the plates are pushed too far apart. Another problem with capacitance strain gauges is an inappropriate scaling of relative sensitivity. In the Hewlett Packard pressure sensor, for example, increasing pressure from a disposable membrane on an external surface reduces the internal capacitance gap, causing a steepening capacitance change at high pressure. This rising sensitivity curve is opposite to what is needed in many applications: a high sensitivity to volume and pressure changes at low pressures, and a declining sensitivity at higher pressures, so that fractional errors remain roughly constant.
The utility of a precision volume sensor with a comparatively high, comparatively linear, and consistently characterized volume/pressure compliance, is apparent from the above discussion and is readily recognized by engineers who deal with the tradeoffs in volume measurement. The practical utility of a frequency output, easily quantified by a frequency or period counter at a microprocessor interface, exceeds the utility of a system requiring an analog/digital conversion in many contexts.